Serotonin (5-hydroxytryptamine, 5HT) mediates a wide variety of central and peripheral psychological and physiological effects through 14 mammalian 5HT receptor subtypes that are grouped into the 5HT1-5HT7 families (Sanders-Bush and Mayer, 2006). The 5HT2 family consists of the 5HT2A, 5HT2B, and 5HT2C membrane-bound G protein-coupled receptors (GPCRs) that signal primarily through Gαq to activate phospholipase (PL) C and formation of inositol phosphates (IP) and diacylglycerol (DAG) second messengers (Raymond et al., 2001). The human 5HT2C receptor (Saltzman et al., 1991) apparently is found exclusively in the brain where it is widely expressed and putatively involved in several (patho)-physiological and psychological processes, including, ingestive behavior (Tecott et al., 1995), cocaine addiction (Fletcher et al., 2002; Rocha et al., 2002; Muller and Huston, 2006), sleep homeostasis (Frank et al., 2002), anxiety (Kennett et al., 1994; Sard et al., 2005; Heisler et al., 2007), depression (Tohda et al., 1989; Palvimaki et al., 1996), epilepsy (Heisler et al., 1998), Alzheimer's disease (Arjona et al., 2002; Stein et al., 2004), motor function (Heisler and Tecott, 2000; Segman et al., 2000), psychosis (Marquis et al., 2007; Siuciak et al., 2007) and response to antipsychotic drugs (Veenstra-VanderWeele et al., 2000; Reynolds et al., 2005). Thus, the importance of the 5HT2C receptor as a pharmacotherapeutic target has been apparent for about 10 years, however, no 5HT2C-specific drugs have been developed.
One challenge regarding drug discovery targeting the 5HT2C receptor is that this GPCR shares a transmembrane domain (TMD) sequence identity of about 80% with the 5HT2A receptor and about 70% with the 5HT2B receptor (Julius et al., 1988; 1990). The highly conserved TMDs and similar second messenger coupling has made development of agonist ligands selective for the 5HT2C receptor especially difficult. Nevertheless, there is compelling evidence that activation of 5HT2C receptors reduces food intake and leads to anti-obesity effects. For example, 5-HT2C knockout mice demonstrate increased feeding and obesity, and, they are resistant to the anorectic effects of d-fenfluramine (Tecott et al., 1995; Vickers et al., 1999; 2001; Heisler et al., 2002). Fenfluramine now is banned, because, although people using the drug showed weight loss due to activation of brain 5HT2C receptors, fenfluramine also activates 5HT2A receptors that may lead to adverse psychiatric (hallucinogenic) effects (Nichols, 2004) and 5HT2B receptors which causes valvular heart disease (Connolly et al., 1997; Fitzgerald et al., 2000; Rothman et al., 2000; Roth, 2007) and pulmonary hypertension (Pouwels et al., 1990; Launay et al., 2002)—fatalities have resulted from the 5HT2B-mediated effects.
Although an agonist ligand truly selective for 5HT2C vs. 5HT2A and/or 5HT2B receptors has not been reported until this paper, it has been possible to partially elucidate the role of brain 5HT2C receptors to attenuate cocaine use and dependence using very selective (i.e., at least 100-fold) 5HT2A and 5HT2C antagonists in rat cocaine self-administration paradigms. For example, the selective 5HT2A antagonist M100907 (Kehne et al., 1996) does not alter responding rate for cocaine self-administration but the selective 5HT2C antagonist SB242084 (Bromidge et al., 1997) increases the rate of cocaine self-administration dose-dependently (Fletcher et al., 2002). The tremendous potential of 5HT2C agonist pharmacotherapy for psychostimulant addiction now is widely recognized (Bubar and Cunningham, 2006).
The pharmacotherapeutic relevance of the 5HT2C receptor in obesity and neuropsychiatric disorders such as psychostimulant addiction has stimulated intense interest by pharmaceutical companies to develop a selective 5HT2C agonist, however, all 5HT2C agonists reported so far also activate 5HT2A and/or 5HT2B receptors (Nilsson, 2006). Nevertheless, the 5HT2 agonist lorcaserin (APD356) recently went to Phase III clinical trials for obesity treatment even though it has only a modest 15-fold selectivity for activation of 5HT2C receptors over 5HT2A receptors (Jensen, 2006; Smith et al., 2006).